Perovskite materials, such as Pr1-xCaxMnO3 (PCMO) demonstrate reversible resistive switching properties, which may be used for low power, low operating voltage, high speed and high density memory applications. When a PCMO film is deposited on a platinum substrate, nano-meter sized PCMO materials exhibit mono-polar switching properties, and crystallized PCMO materials exhibit bipolar switching properties. However, when a PCMO film is fairly uniformly crystallized the high/low (RH/RL) bipolar switching ratio is very low. An asymmetric material is required for high speed bipolar switching application. Although oxygen density distribution and device geometry manipulation are able to achieve good bipolar switching property for larger devices, these processes are difficult to scale down to very small memory cell size.
The resistance of a perfectly uniform Electric Pulse-Induced Resistance (EPIR) variation material memory resistor can only be programmed by using a narrow electrical pulse to set the resistance to a high resistance state, RH, and using a wide electrical pulse to reset the resistance to a low resistance state, RL. Various methods, such as tailoring crystalline structure, oxygen content distribution, and device geometry design, have been proposed to achieve these goals. A large difference in crystalline structure is required, which is difficult to scale to a very thin film application for use in very small size memory devices. The oxygen content distribution requires a higher oxygen content at the upper portion of the memory thin film. This may be achieved easily by an annealing process, however, because oxygen is mobile in RRAM material, such as PCMO, reliability issues arise when the temperature of the device is raised, either during device fabrication processes or during circuit operation.
U.S. patent application Ser. No. 10/831,677, filed Apr. 23, 2004, of Zhuang et al., entitled PCMO Thin Film with Memory Resistance Properties, describes basic fabrication of a PCMO thin film.
U.S. Pat. No. 6,939,724 B1, granted Feb. 17, 2005, to Zhuang et al., entitled Method for Obtaining Reversible Resistance Switches on the PCMO Thin Film Integrated with a Highly Crystallized Seed Layer, describes use of a seed layer of highly crystallized PCMO, which is deposited by MOCVD. A second PCMO thin film is deposited by spin-coating on the seed layer, which produces a polycrystalline layer, wherein the second PCMO layer is ten times thicker than the seed layer. The two step PCMO deposition process allows formation of a combined PCMO layer which has a higher resistance potential than does the PCMO layers of the known prior art.
U.S. patent application Ser. No. 10/836,689, filed Apr. 30, 2004, of Li et al., entitled PCMO Thin Film with Resistance Random Access Memory (RRAM) Characteristics, describes PCMO thin films having predetermined memory-resistance characteristics and describes fabrication processes. The method includes forming a Pr3+1-xCa2+xMnO thin film composition, where 0.1<x<0.6, and where the ratio of Mn and O ions are varied as follows: O2−(3±20%); Mn3+((1−x)±20%); and, Mn4+(x±20%). When the PCMO thin film has a Pr3+0.70Ca2+0.30Mn3+0.78Mn4+0.22O2−2.96 composition, the ratio of Mn and O ions varies as follows: O2−(2.96); Mn3+((1−x)+8%); and, Mn4+(x−8%).